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Alessandra Di Cecco Collaborators: A. Calamida, M. Monelli, P.B. Stetson, A. R. Walker

Tor Vergata Astrophysics. Observatory of Rome. Absolute age of the old metal-poor GC M92. Alessandra Di Cecco Collaborators: A. Calamida, M. Monelli, P.B. Stetson, A. R. Walker Roma: G. Bono, R. Buonanno, C.E. Corsi, I. Ferraro, G. Iannicola, L. Pulone

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Alessandra Di Cecco Collaborators: A. Calamida, M. Monelli, P.B. Stetson, A. R. Walker

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  1. Tor Vergata Astrophysics Observatory of Rome Absolute age of the old metal-poor GC M92 Alessandra Di Cecco Collaborators: A. Calamida, M. Monelli, P.B. Stetson, A. R. Walker Roma: G. Bono, R. Buonanno, C.E. Corsi, I. Ferraro, G. Iannicola, L. Pulone Pisa: R. Becucci, S. Degl’Innocenti, P.G. Prada Moroni Teramo: A. Pietrinferni, S. Cassisi

  2. Summary • Brief introduction to Galactic Globular Clusters (GGCs) • Goals: • Age of a GGC • Multi-populations (previous talk by P. Ventura) • Star counts and evolutionary lifetimes • Conclusions Data and reduction strategy Analysis and results

  3. Galactic Globular Clusters (GGCs) History: Johann Abraham Ihle in 1665 discovered the first GGC, M22. Today, we know more than 150 GGCs. General Features: Most GGCs contain 105 to 106 coeval stars with the same chemical composition (Simple Stellar Population, SSP) Metallicity range: Z= 10-2 -10-4 GCs are the oldest known components of the Galactic halo (Baade 1950) Globular Cluster GCs provide independent constraints: on the age of the Universe on the evolutionary theory of stellar structures on the formation of the Galactic spheroid on the primordial Helium content

  4. ω Centauri 10Gyr isocrhones (Maraston 2005) 1.Age and 2.Multi-populations Isochrones are functions of Initial Helium abundance (Y), metallicity (Z) [Fe/H]=Log(N(Fe)/N(H)) * - Log(N(Fe)/N(H))סּ [α/Fe] ∑(O,Ne,Mg,Si,Ar,S,Ca,Ti) [M/H]=[Fe/H]+Log(10^([α/Fe])*0.638+0.362) (Salaris et al. 1993) Age is affected by Distance Modulusand Reddening Some GGCs cannot be explained as a SSP: • ω Centauri has a triple split of the RGB and a double MS split(Pancino et al. 2000, Sollima et al. 2005, Bedin et al. 2004, Norris 2004). • NGC2808 shows triple MS splitting(Piotto et al. 2007) • NGC1851 and NGC6388 have double sub-giant branch: CN-strong/weak (Milone et al. 2008, Cassisi et al.2008) • The anticorrelation (CNONa) is not predicted by the canonical stellar models (AGB, fast-rotating stars) (Gratton et al. 2006, D’Ercole et al. 2008)

  5. M92 (NGC6341) RA:17 17 07 Declination:+43 08 11 Why M92? • Extremely metal-poor ([Fe/H]=-2.32±0.07,Kraft & Ivans 2004) • It is far from the Galactic Plane (E(B-V)=(0.02-0.03)±0.01, Zinn 1980, Reed et al. 1988) • Its distance modulus was widely investigated (DM=14.65±0.09, Del Principe et al. 2005, 2006; Solima et al. 2006) • The CMD does not present peculiar/anomalous features • HOWEVER -- Spectroscopic measurements show evidence of strong C (~3) and N (~10) variations in evolved SGB, RGB, AGB stars. (Carbon et al. 1982) Large field-of-view High spatial resolution to overcome the central crowding Two Datasets • Large Mosaic CCDs, i.e. MegaCam at the Canada-France-Hawaii Telescope (CFHT) • 2. Advanced Camera for Surveys (ACS)

  6. 1 2 3 4 5 6 7 8 9 14.4’ 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 6.4’ MegaCam CFHT Camera MegaCamconsists of 36 CCDs (2048x4612 pxl) Field-of-view: 1°x1° Plate scale: 0.187’’/pxl Filters: a set of SDSS bands (u*,g’,r’,i’,z’) M92 data:57 images, deep and shallow exposure times Reduction strategy: Elixir pre-processed data. PSF photometry was performed using DAOPHOTII, ALLSTAR, ALLFRAME programs (Stetson 1987,1994). Local secondary standard sequence provided by Clem et al 2007.

  7. Many background galaxies Reflection

  8. ACS Space Data Field-of-view: 3’x3’ One image for each filter : F814W-F606W, F814W-F475W We calibrated F814W  i’ (I), F606W  r’ (V/R) , F475W  g’ (B) The final catalogue (CFHT+ACS) consists of ~140’000 stars

  9. Theoretical Comparison • The isochrones were provided by Pisa Library (atmosphere models provided by Castelli & Kurucz 2006, diffusion): • [a/Fe]=+0.4, [Fe/H]=–2.32 ( Kraft & Ivans 2004) • E(B-V) and DM0 within the uncertainties:μ=DM0=14.65±0.09 E(B-V)=(0.02-0.03)±0.010 σ (g’=20)=0.02 • Theoretical comparison: • Age of 11 ± 1 Gyr confirmed by further analysis performed in ACS and Johnson bands. • Limited metallicity-[distance-reddening] degeneracy: we found a similar age using [Fe/H]=-2.01 but this Fe-abundance is only marginally in agreement with Fe measurement [Fe/H]=-2.15±0.07 by Carretta & Gratton (1997). • No evidence of multi-population

  10. 3. Star count ratios and lifetimes Post MS evolutionary phase lifetimes (t) are directly related to the star counts (N) N iαt , N i /N j= ti/tj Omega Centauri (Castellani et al 2007) Changing in HB luminosity function RG/MS agrees with predicted lifetime HB/RG is higher than predicted values ~30-40% NGC2808 R-parameter decreases inward (Castellani et al. 2006): R=NHB/NRG R parameter is related to the He abundance. Iannicola et al. (2008) : the culprits are the RGB, since they decrease outwards

  11. Star count ratios in M92 We studied the ratios between HB, RG and MS stars along the radial distance (rg) Selected regions for star counts in red Theoretical lifetimes Star count ratios (Kurucz, 0.75Mo,,[Fe/H]=-2.32)We found good agreement between theoretical HB/RGB = 0.20±0.09 0.28±0.04 lifetimes and star count ratios RGB/MS = 0.44±0.03 0.38±0.04 We found that each ratio assumes constant value moving HB/MS = 0.09±0.05 0.10±0.02 from the innermost to the outermost regions of M92.

  12. Conclusions We have obtained multiband photometry of GGC M92 using MegaCam and ACS and… ..we found an age of 11±1 Gyr (marginally affected by metallicity-[distance-reddening] degeneracy) ..we did not find multi-populations ..we investigated star count ratios and we found good agreement with theoretical lifetimes. Moreover, the ratios are constant as a function of the radial distance Work in progress: We plan to investigate the metallicity of M92 using an internal metallicity indicator  RGB bump

  13. Relation between [M/H] and V(bump-HB) The ‘first dredge-up’ occurs when sinking of the convective envelope reaches the thin H-burning shell. The ‘RGB bump’ occurs when H-burning shell approaches the H-discontinuity left by over the first dredge-up. In the luminosity range where the RGB bump takes place the RG stars spend a longer time interval. RGB BUMP The metallicity of a GCs is related to the V(bump)-V(HB)

  14. Thank you for your attention!

  15. Degeneration 1° Agreement: [Fe/H]=–2.32, DM(μ)=14.74, E(B-V) CFHT=0.035 [E(B-V)ACS=0.025] 2° Agreement: [Fe/H]=–2.01DM(μ)=14.70, E(B-V) CFHT=0.030 [E(B-V)ACS=0.018]

  16. State of the Art

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